Learn Hardcore Latin: Aper per silvam currit. - The wild boar runs through the forest.

Here, aper means boar and is the thing doing the action of running. So aper is the subject, and currit agrees with it as he/she/it runs.

Why is it silvam and not silva?

Because the preposition per takes the accusative case.

So:

silva = nominative/ablative singular
silvam = accusative singular

Since the sentence says per silvam = through the forest, Latin uses the accusative form silvam after per.

Is silvam the direct object of currit?

No. Even though silvam is accusative, it is not the direct object here.

It is the object of the preposition per. So in per silvam, the whole phrase means through the forest, and silvam belongs with per, not directly with currit.

This is important because in Latin, accusative does not always mean direct object.

What does currit tell me?